It sounds like a factory farm legend. One day, the manure collected under the floorboards of a pig barn starts foaming uncontrollably. Then, the foam releases gas that explodes. In one incident — all too real — over 1,500 pigs were killed, and one human seriously injured. What's going on?

As Sarah Zhang explains in a great essay over at Nautilus, the answer is complicated. Something is turning pig shit into explosives, and it's happened recently. Famers have been using the same barn designs for a long time, built with slatted floors that allow the pig manure to fall through cracks and form a pile under the structure. Only in the last five years had they ever seen the manure foam up. So what's changed, and why is it explosive? Zhang writes:

As a matter of physics, every foam needs three components: gas, stabilizer, and surfactant. Manure is normally teeming with microbes, which produce gases like methane and whose cells can act as stabilizers. Therefore the surfactant—any chemical such as soap that lowers the surface tension of liquids to allow bubbles to form—is a likely candidate for what's new.

Around the same time that foaming manure became a problem, farmers also started feeding their pigs more and more distiller's dried grains with solubles (DDGS), a cereal-like byproduct of ethanol production. DDGS is full of plant fibers and long-chain fatty acids—plausible surfactant candidates. Since the US provides huge subsidies for making ethanol from corn, DDGS was cheap and plentiful source of animal feed. It makes economic sense, at least.

DDGS has been a suspected culprit of foaming manure from the beginning (one magazine called it a "pig bang theory"), but experiments are still inconclusive.

So a new kind of pig feed may be responsible for the foaming, and two gasses in the foam — methane and hydrogen sulfide — are turning barns into gas explosions waiting to happen. But the problem, as Zhang points out, is that it's very hard to determine whether DDGS is actually the foam-creating culprit. There are too many other factors at play, and the barns themselves are usually piled with manure that's over a year old. That makes it hard to test for novel chemicals.

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Currently, farmers are solving the problem by feeding pigs an antibiotic that seems to break down some of the microbes that cause foaming manure. Which means we're once again pumping antiobiotics into the environment at a massive scale, which breeds more antibiotic resistant bacteria.

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Downstream from all of this, humans will be profoundly affected. As more antibiotic-resistant bacteria reproduce in the environment, the more likely we are to be killed or gravely harmed by these microbes when we get infections. Because we've been using antibiotics to prevent foaming manure, we'll have bred our own bacterial doom.

This solution to the exploding big barn is yet another example of environmental engineering that's being done without an understanding of the complexity of the machine that we call Earth. Instead of changing what the pigs eat — or, indeed, changing how they are housed so that they aren't living in their own shit — farmers are adding new inputs to this system that will eventually create an output of bacteria that humans cannot defend against.

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If you want to know how the next wave of deadly bacterial infections might start, you can look to shortsighted solutions like the ones we're seeing on pig farms. Instead of removing the cause of the problem — piles of manure — we're creating an even more deadly problem down the line.